Quark–gluon plasma

Quark–gluon plasma is an extremely hot, dense state of matter in which quarks and gluons are no longer confined inside individual hadrons. It existed in the early universe and can be created briefly in high-energy nuclear collisions.

Quantum chromodynamics: The theory of the strong interaction among quarks and gluons. Its changing coupling with energy governs how quarks and gluons become deconfined.
Relativistic heavy-ion collider: A Brookhaven National Laboratory accelerator that collides heavy nuclei at relativistic energies. Its gold-ion collisions provide a major experimental setting for producing and studying the plasma.
Big Bang: The early expansion of the universe from a hot, dense state. The universe passed through a quark–gluon plasma phase shortly after its beginning.
Quark–gluon plasma in the early universe: The primordial quark–gluon plasma phase that filled the universe before hadrons formed. It shares the plasma’s basic state but differs from the tiny, rapidly expanding laboratory droplets.
Color confinement: The phenomenon that prevents isolated quarks and gluons from appearing at ordinary energies. The plasma is the high-temperature regime in which confinement no longer binds them into separate hadrons.
Large Hadron Collider: A particle accelerator at CERN that collides protons and heavy ions. Lead-ion collisions there create hotter plasma and enable measurements complementary to RHIC.
Early universe: The universe’s first stages, when it was far hotter and denser than it is today. Its cooling transformed primordial plasma into the hadrons that later formed matter.
Hadron gas: A hot medium whose constituents are hadrons rather than deconfined quarks and gluons. It represents the confined phase adjacent to quark–gluon plasma.
Asymptotic freedom: The decrease of the strong interaction’s effective strength as quarks approach one another or energy rises. It helps explain why interactions inside the plasma become weaker at sufficiently high temperatures.
Heavy-ion collision: A high-energy collision between atomic nuclei, often used to produce strongly interacting matter. The short-lived plasma is formed in the hottest central region of these collisions.

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